Fluid catalytic cracking has undergone many changes since its inception in the early 1940's. One of the recent process advances in fluid catalytic cracking has been the advent of zeolite catalysts usage which has prompted many process design modifications. Feedstocks, however, have changed very little up to recent years, being comprised of mostly atmospheric and vacuum gas oils. However, with the economic need of the industry to process poorer quality crude oils, and a need at the same time to maintain high gasoline yields, the ability to effect conversion of poor quality feed stocks in a FCCU process is now of significant importance.
The present invention is directed to extending the processed boiling range of crude oil feedstocks to include substantially all of the atmospheric bottoms, a residual or reduced crude portion thereof and comprising vacuum tower bottoms by catalytically cracking such materials in a selective process more fully discussed below that converts a highly atomized-vaporized mixture of the feed components at a relatively high temperature with an ultrastable crystalline zeolite catalyst herein identified.
U.S. Pat. No. 4,287,048 particularly identifies an ultrastable "Y" type crystalline zeolite and its method of preparation in the following manner. "Stabilized" or ultrastable Y-type zeolites are well-known. They are described, for example, in U.S. Pat. Nos. 3,293,192 and 3,402,996 and the publication Society of Chemical Engineering (London) Monograph Molecular Sieves, page 186 (1968) by C. V. McDaniel and P. K. Maher, the teachings of which are hereby incorporated by reference. In general, "ultrastable" refers to a Y-type zeolite which is highly resistant to degradation of crystallinity by high temperature and steam treatment and is characterized by a R.sub.2 O content (wherein R is Na, K or any other alkali metal ion) of less than 4 weight percent, preferably less than 1 weight percent and a unit cell size less than 24.5 Angstroms and a silica to alumina mole ratio in the range of 3.5 to 7 or higher. The ultrastable form of Y-type zeolite is obtained primarily by a substantial reduction of the alkali metal ions and the unit cell size reduction. The ultrastable zeolite is identified both by the smaller unit cell and the low alkali metal content in the crystal structure.
As is generally known, the ultrastable form of the Y-type zeolite can be prepared by successively base exchanging a Y-type zeolite with an aqueous solution of an ammonium salt, such as ammonium nitrate, until the alkali metal content of the Y-type zeolite is reduced to less than 4 weight percent. The base exchanged zeolite is then calcined at a temperature of 1000.degree. F. to 1500.degree. F., for up to several hours, cooled and thereafter again successively base exchanged with an aqueous solution of an ammonium salt until the alkali metal content is reduced to less than 1 weight percent, followed by washing and calcination again at a temperature of 1000.degree. to 1500.degree. F. to produce an ultrastable zeolite Y. The sequence of ion exchange and heat treatment results in the substantial reduction of the alkali metal content of the original zeolite and results in a unit cell shrinkage which is believed to lead to the ultra high stability of the resulting Y-type zeolite. The particle size of the zeolites is usually in the range of 0.1 to 10 microns, more particularly in the range of 0.5 to 3 microns.
Suitable amounts of the ultrastable Y-type zeolite in the catalyst of the present invention include from about 10 to about 80 weight percent, preferably from about 30 to about 50 weight percent, based on the total catalyst.